This invention relates to improvements in an apparatus for controlling an A.C. elevator.
Recently, owing to the progress of power electronic elements as well as the technology for controlling them, an apparatus wherein an induction motor is supplied with variable-voltage and variable-frequency A.C. power by the use of an inverter so as to drive a cage while performing a speed control has been proposed in, for example, `ELEVATOR WORLD (published by ELEVATOR WORLD, INC.)`, DECEMBER 1982, pp. 63-67, "OTIS UNVEILS VARIABLE FREQUENCY DRIVE", and is shown in FIG. 1.
Referring to the figure, numeral 1 designates a single-phase A.C. power source, to which a charging current-limiting resistor 2 is connected. Numeral 3 designates a single-phase full-wave rectifier circuit which is composed of diodes 3A-3D connected to the A.C. power source 1 through the resistor 2 and which converts single-phase alternating current into direct current. Numeral 4 indicates batteries which are connected on the D.C. side of the rectifier circuit 3. Shown at numeral 5 is an inverter of the well-known PWM type which is composed of transistors 5A-5F (or gate turnoff transistors) connected to the batteries 4. Every two of the transistors 5A-5F connected in series, diodes 5a-5f are connected in parallel therewith. The inverter 5 inverts a fixed D.C. voltage into a variable-voltage and variable-frequency A.C. voltage by pulse width control. A three-phase induction motor 6 is connected on the A.C. side of the inverter 5. A driving sheave 7 for a hoist is driven by the motor 6 A main rope 8 is wound round the sheave 7. Connected to the main rope 8 are a cage 9 and a balance weight 10.
Owing to the D.C. power converted by the rectifier circuit 3, a fixed quantity of charging current flows into the batteries 4. When the cage 9 is to move, the transistors 5A-5F of the inverter 5 turn "on" successively in accordance with the running direction of the cage, to generate the variable-voltage and variable-frequency A.C. power in a phase sequence corresponding to the running direction. Thus, the motor 6 starts in a direction determined by the phase sequence of its inputs, and the cage 9 begins to run. Under a certain running state of the cage 9, however, the motor 6 produces regenerative power, which is restored to the power source side through the inverter 5. That is, the charges having been stored in the batteries 4 are discharged to the motor 6 through the inverter 5, or conversely the batteries 4 are charged through the inverter 5.
FIG. 2 illustrates an example of such operation, that is, the charged and discharged states of the batteries 4 in the case where heavy load ascent and descent operations have successively arisen. In the figure, T.sub.1 indicates the period of time of the heavy load ascent operation, and T.sub.2 the period of time of the heavy load descent operation. t.sub.1 denotes an acceleration time interval, t.sub.2 a constant-speed running time interval, and t.sub.3 a deceleration time interval. Similarly, t.sub.4 -t.sub.6 denote an acceleration time interval, a constant-speed running time interval and a deceleration time interval, respectively.
During the acceleration time interval t.sub.1 of the heavy load ascent operation, the motor 6 consumes high power, and hence, the quantity of discharge is large. The power consumption is next to the above during the constant-speed running time interval t.sub.2, and power is scarcely consumed during the deceleration time interval t.sub.3. During the acceleration time interval t.sub.4 of the heavy load descent operation, the cage is accelerated by the force of gravity based on the heavy load thereof, and hence, the charges of the batteries 4 are scarcely discharged. The batteries are somewhat charged by the regenerative power during the constant-speed running time interval t.sub.5, and are considerably charged during the deceleration time interval t.sub.6. As illustrated in FIG. 1, the batteries 4 are charged by the A.C. power source 1 at all times. However, when they are charged by the regenerative power, a great charging current cannot be caused to flow because of the prevention of the overcharge thereof. Thus, the charges in the batteries 4 change as Q.sub.0 .fwdarw.Q.sub.1 .fwdarw.Q.sub.2 as illustrated by a curve QA in FIG. 2. The repetition of such abrupt charge and discharge is peculiar to the batteries 4 for the elevator controlled by the inverter.
The repetition of the charge and discharge, however, seriously affects the lifetime of the batteries 4. The shortening of the lifetime due to the charge and discharge is fatal to an apparatus for use in an equipment such as the elevator which must guarantee a long lifetime (15-20 years), and it has hindered putting the apparatus into practical use.